This invention can be characterized in the general field of refractory compositions which contain niobium and silicon.
Turbines and other types of high-performance equipment are designed to operate in a very demanding environment. In a typical gas turbine engine, air is compressed in a compressor, and mixed with fuel and ignited in a combustor, for generating hot combustion gases. The gases flow downstream through a high pressure turbine (HPT) having one or more stages, including a turbine nozzle and rotor blades. The gases then flow to a low pressure turbine (LPT) which typically includes multi-stages with respective turbine nozzles and rotor blades.
Choice of a particular metal to be used in a gas turbine engine depends in large part on the projected temperature-exposure of the engine component, along with other specified requirements—strength, creep resistance, oxidation resistance, environmental resistance, weight requirements, and the like. Nickel-based superalloys are often the materials of choice for the “hot” sections of the turbine, where metal temperatures as high as about 1150° C. are typical. Titanium alloys, which are lighter than the nickel alloys, are often used in the compressor sections of the turbine engines, where temperatures are lower, e.g., less than about 600° C.
Although nickel-based superalloys are still the standard for many turbine components, the desire for materials with even higher temperature capability has been described in many sources. Examples of these materials are the refractory metal intermetallic composites (RMIC's). Many of these are based on niobium (Nb) and silicon (Si), and are described, for example, in U.S. Pat. No. 5,932,033 (Jackson and Bewlay); U.S. Pat. No. 5,942,055 (Jackson and Bewlay); and U.S. Pat. No. 6,419,765 (Jackson, Bewlay, and Zhao). These materials usually have a multi-phase microstructure, and possess a number of very desirable properties. For example, they often combine high-temperature strength, low-temperature toughness, and relatively low density, as compared to many nickel alloys. Moreover, the RMIC's often have melting temperatures of up to about 1700° C. For these reasons, such materials are very promising for use in applications in which the temperatures exceed the current service limit of the nickel-based superalloys.
The gas turbine engines mentioned above include a number of components, each of which is exposed to a different environment during operation. Thus, each component often has different requirements, in terms of strength, creep resistance, oxidation resistance, toughness, fracture resistance, fatigue resistance, wear resistance, and the like. While many RMIC materials are often superlative in one or two of these characteristics, they may not always meet the specification for other characteristics.
As an illustration, the composites may generally possess some beneficial mechanical and chemical properties, but they may not adequately balance oxidation resistance with strength, toughness and creep resistance. As a specific example, the constituents of an RMIC-based airfoil material can be adjusted to increase oxidation resistance, but often at the expense of strength and creep performance. As another illustration, some promising RMIC composites exhibit good strength, low density, and high stiffness at elevated temperatures, e.g., above about 1000° C. However, those same materials may exhibit inadequate damage tolerance and very low fracture toughness at lower temperatures (about 600° C.-1000° C.), e.g., the temperature conditions often associated with low-pressure turbine sections.
In view of the discussion above, it should be apparent that additional niobium-silicide alloys which exhibit an improved balance of properties for selected temperature-based applications would be welcome in the art. In general, the materials should exhibit good low-temperature toughness and good high temperature strength and creep resistance. Materials which provide good performance at the intermediate operating temperatures, e.g., about 600° C.-1000° C., would also be of considerable interest. Moreover, the materials should be lighter than many of the nickel-based superalloys which are commonly employed for turbine components.